i tried breeding before... well, I think it depends on luck.

If your pair is a bonded pair, they will mate when the time is right. Your cage has to be big in order for your tiels to feel comfortable. Also, providing a large next box that is able to find both birds is good but not too big till the female can get seperated from her eggs. Once your tiels get used to their large cage and nest box, they will start breeding.

Provide lots of fresh foods, parrot food and cuttle fish bone or calcium block.